Straddle carriers are employed in harbors and container terminals to transport and stack containers. They are usually steered hydraulically, that is to say, their diesel engine drives a hydraulic pump that forces oil out of a tank via control valves into hydraulic cylinders which use a steering linkage to make the wheels of the vehicle move into certain steering positions.
A straddle loader with an electric wheel drive is described in German publication DE 203 11 886.3, the entire disclosure of which is incorporated by reference herein, although no mention is made of the steering of the straddle loader.
The hydraulic steering consists of a hydraulic system with a pump, cylinder, valve block, oil tank and tubing on the vehicle.
So-called steering force simulators are known for the steering of such straddle carriers. They simulate an increasing force on the operator's steering wheel when the travel on the steering wheel becomes excessively large in comparison to the actual displacement of the wheels in order to call attention to the occurrence of over-steering. Steering force simulators are available in a purely hydraulic version or else with electric solenoid brakes. Owing to the structure of the straddle carrier and to the arrangement of the operator's cab, purely hydraulic steering systems of the type known for straddle carriers have the drawback that many tubes and hoses are needed which, on the one hand, cause a considerable delay and, on the other hand, are a source of frequent malfunctions. Consequently, purely hydraulic steering systems are not practical for straddle carriers. The disadvantage of the known electric systems is that the steering force is generated by means of a solenoid brake with a friction lining. This friction lining has the drawback that the wear and tear on the brake lining causes a change in the braking torque so that readjustments have to be made frequently. Thus, maintenance work has to be carried out in a workshop, which restricts the availability of the vehicle. Another disadvantage of such a system is that, when the operator reverses the direction of movement, slack or play occurs in the steering force simulator which does not affect the steering operation but it does give an unpleasant steering feel to the operator. The same type of slack is also known with conventional hydraulic steering systems.